Management of basilar invagination (BI) and atlanto axial dislocation (AAD) is complex. The traditional paradigms for management included a trans oral excision of odontoid process followed by posterior instrumented fusion1,2,3. Wang et al suggested a trans-oral release of the ligaments around the odontoid process as the first stage of the treatment, followed by a posterior instrumented fixation in a second surgery.
Distraction of the C1-C2 joint has been recognized as an established form of treatment over the past decade4, 5. Distraction of C1-C2 joint can effectively reduce the BI and also AAD to some extent. However the main shortcoming of distraction is that it can provide re-alignment in a vertical direction mostly and does not offer any movement in the horizontal axis.
Jian et al introduced a concept of intra-operative distraction cases of BI with assimilated C1 arch, where a rod was connected to C2 pedicular screw and occipital screw following, which distraction was performed reducing both BI and AAD. They achieved satisfactory results. However, the shortcoming of this procedure was that, it could provide distraction only as a method of reduction for both AAD and BI. AAD for its optimal reduction also requires a forward movement of dens as compared to BI, which requires only a vertical distraction. This is reflected in their results, where BI could be reduced in almost all patients but the AAD could be reduced completely in only 85% of their cases. In addition, distraction only without a spacer placement carries a risk of re-settling, this also was reflected in some of their cases.
Hsu W et al overcome this shortcoming by describing a novel technique in 2 cases of acquired (one infection and other in metastasis) occipito-cervical instability. Here, apart from intra-operative occipiti-cervical distraction, they also provided an extension of neck by applying compression between the upper occipital screw and another screw tightened more superiorly on the rod, which technique clearly demonstrated that while distraction corrects BI, extension while maintaining distraction results in correction of AAD. Distraction was performed without a spacer followed by extension that was provided by compressing 2 cranial screws. The latter technique while useful in acquired destructive pathologies (like malignancies) of craniovertebral junction may be difficult in developmental anomalies with more rigid joints. In addition, resettling may occur over a period of time due attrition at the bone screw interface.
In the Sonntag technique, a sublaminar cable is passed under the posterior C1 arch from inferior to superior. Next a notched iliac crest is placed in between the spinous process of C2 and wedged underneath the posterior arch of C1. Both the superior aspect of the C2 spinous process and the inferior arch of C1 are decorticated before graft placement. The cable is then looped over the iliac crest autograft and placed into a notch created on the inferior aspect of the C2 spinous process. The cable is then tightened and crimped.
The disadvantage of this method involves the use of a halo to immobilize patients for three months after surgery and the use of a rigid cervical collar for an additional one to two months after that. Neurological complication including quadriparesis can occur in up to 5 to 7% of cases and breakage of wire might occur.
The technique of segmental atlantoaxial fixation and fusion using C1 lateral mass screw and C2 pedicle screw and plates was pioneered by Prof Goel et al. The main drawback of this procedure is that only vertical distraction (which corrects BI) is provided in this technique and no horizontal corrective motion is provided for in this technique, which would be required to correct the AAD. In addition, C1 lateral mass screws joined with C2 pars screws while providing stabilization, may not provide enough forces to resist any intra-operative manipulation that may be carried out in view of the short lever arm with respect to the fulcrum. The procedure is technically demanding and precise and an exact three-dimensional understanding of the anatomy of the region and of the vertebral artery is mandatory. Large venous plexuses in the lateral glutter need to be handled appropriately.